It has long been known that the paint adhesion and post-coating corrosion resistance can be improved through chemical treatments in which a heavy metal and/or compound thereof is deposited on the surface of zinc coated steel sheet. Examples from the art in this regard are (1) Japanese Patent Publication Number 52-22618 (22,618/77] and (2) Japanese Patent Publication Number 43-12974 [12,974/68]. Both of these concern the deposition of a heavy metal, such as nickel, cobalt, iron, or the like, onto galvanized steel sheet. Heavy metal or oxide thereof is deposited onto zinc coated steel sheet by treatment in the acidic range at pH 2 according to the invention of reference (1) and by treatment in the alkaline range at a pH of at least 11 according to the invention of reference (2). A disadvantage accruing to each of these prior art references is the occurrence of excessive etching of the zinc (an amphoteric metal) by the H.sup.+ ion or OH.sup.- ion. This results in a degradation in the performance of the end product, for example, a decline in corrosion resistance.
Thus, methods are already known for improving the adhesion of coatings (e. g., paints, laminates, ceramic coatings, etc.) to the substrate through the substitution precipitation of cobalt or nickel onto zinc coated steel sheet, but in each case the prior treatment solution, being acidic or alkaline, etches the zinc substrate too much, with a resulting decline in performance, for example, in corrosion resistance. On the other hand, while excessive etching does not occur in the neutral pH region, the corresponding rate of metal substitution precipitation is so slow as to be useless for practical applications.